Population Change Can Influence Climate Change

Changes in population, including aging and urbanization, could affect global carbon dioxide emissions

The Impact of People Upon the Planet

Changes in the human population, including aging and urbanization, could significantly affect global
emissions of carbon dioxide over the next 40 years.

These findings appear in a paper in the journal Proceedings of the National Academy of
Sciences (PNAS), were recognized by scientists at the National Center for Atmospheric Research (NCAR), the
International Institute for Applied Systems Analysis (IIASA) and the National Oceanographic and Atmospheric
Administration (NOAA).

Their work was funded by the National Science Foundation (NSF), a European Young Investigator's Award, and the Hewlett Foundation.

"By examining the relationship between population dynamics and greenhouse gas emissions, this
groundbreaking research increases our understanding of how human behaviors, decisions and lifestyles
will determine the path of future climate change," says Sarah Ruth, program director in NSF's Division
of Atmospheric and Geospace Sciences, which funds NCAR.

Three Billion More Urban Dwellers by 2050?

By mid-century it is estimated that global population could rise by more than three billion people, with most
of that increase occurring in urban areas.

The study showed that a slowing of that population growth could contribute to significantly reducing
greenhouse gas emissions.

Slowing Population Growth

The researchers found that if population follows one of the slower growth paths foreseen as plausible by
demographers at the United Nations, by 2050 it could account for 16 to 29 percent of the emission reductions
thought necessary to keep global temperatures from causing serious impacts.

The effect of slower population growth on greenhouse gas emissions would be even larger by the end of the century.

"If global population growth slows down, it is not going to solve the climate problem, but it can make a
contribution, especially in the long term," says the paper's lead author, Brian O'Neill, an NCAR scientist.

Where Population Grows is Significant

O'Neill's co-author, IIASA scientist Shonali Pachauri, says that slower population growth will have different
influences, depending on where it occurs.

"A slowing of population growth in developing countries today will have a large impact on future global population
size. However, slower population growth in developed countries will matter to emissions, too, because of higher per
capita energy use," says Pachauri.

Scientists have long known that changes in population will have some effect on greenhouse gas emissions, but there
has been debate on how large that effect might be.

The researchers sought to quantify how demographic changes influence emissions over time, and in which regions
of the world. They also went beyond changes in population size to examine the links between aging, urbanization, and emissions.

The Unique Impact of Urban Populations

The team found that growth in urban populations could lead to as much as a 25 percent rise in projected carbon
dioxide emissions in some developing countries.

The increased economic growth associated with city-dwellers was directly correlated with increased emissions, largely
due to the higher productivity and consumption preferences of an urban labor force.

The Unique Impact of Aging Populations

In contrast, aging can reduce emissions levels by up to 20 percent in some industrialized countries. Older populations
are associated with lower labor force participation, and the resulting lower productivity leads to lower economic growth.

"Demography will matter to greenhouse gas emissions over the next 40 years," says O'Neill. "Urbanization will be
particularly important in many developing countries, especially China and India, and aging will be important in industrialized countries."

The researchers worked with projections showing that population aging will occur in all regions of the world, a
result of people living longer and declines in fertility.

Modeling the Impact of Demographic Change

The authors developed a set of economic growth, energy use, and emissions scenarios, using a new computer model
(the Population- Environment-Technology model, or PET).

To capture the effects of future demographic change, they distinguished between household types, looking at age,
size, and urban vs. rural location.

The Unique Impact of Household Types

In addition, they drew on data from national surveys covering 34 countries and representative of 61 percent of the
global population to estimate key economic characteristics of household types over time, including labor supply and demand for consumer goods.

"Households can affect emissions either directly, through their consumption patterns, or indirectly, through their
effects on economic growth," O'Neill explains.

The authors also suggest that developers of future emissions scenarios give greater consideration to the implications
of urbanization and aging, particularly in the U.S., European Union, China and India.

The Range of Future Energy Demand

"Further analysis of these trends would improve our understanding of the potential range of future energy demand and
emissions," says O'Neill.

The researchers caution that their findings do not imply that policies affecting aging or urbanization should be implemented
as a response to climate change, but rather that better understanding of these trends would help anticipate future changes.

The number of people on the planet influences the rate of climate change. However, where those people live, their age and their household type can have a greater impact than the number of people alone. Young, urban people in single-person households in an industrialized country have a much different impact than older people living in multi-person rural households in a undeveloped country. NASA image - Satellite View of Earth at Night..

The people living in the African interior have a very different climate impact when compared to those who live in a major European city. NASA Image.